1. Field of Invention
The present invention relates to an apparatus and method for perforating well casing and/or a subterranean formation(s), and more particularly, to such an apparatus and process wherein a propellant is employed to substantially simultaneously enhance the effectiveness of such perforations and to stimulate the subterranean formation(s).
2. Description of Related Art
Individual lengths of relatively large diameter metal tubulars are secured together to form a casing string which is positioned within a subterranean well bore to increase the integrity of the well bore and provide a path for producing fluids to the surface. Conventionally, the casing is cemented to the well bore face and subsequently perforated by detonating shaped explosive charges. These perforations extend through the casing and cement a short distance into the formation. In certain instances, it is desirable to conduct such perforating operations with the pressure in the well being overbalanced with respect to the formation pressure. Under overbalanced conditions, the well pressure exceeds the pressure at which the formation will fracture, and therefor, hydraulic fracturing occurs in the vicinity of the perforations. As an example, the perforations may penetrate several inches into the formation, and the fracture network may extend several feet into the formation. Thus, an enlarged conduit can be created for fluid flow between the formation and the well, and well productivity may be significantly increased by deliberately inducing fractures at the perforations.
When the perforating process is complete, the pressure within the well is allowed to decrease to the desired operating pressure for fluid production or injection. As the pressure decreases, the newly created fractures tend to close under the overburden pressure. To ensure that fractures and perforations remain open conduits for fluids flowing from the formation into to the well or from the well into the formation, particulate material or proppants are conventionally injected into the perforations so as to prop the fractures open. In addition, the particulate material or proppant may scour the surface of the perforations and/or the fractures, thereby enlarging the conduits created for enhanced fluid flow. The proppant can be emplaced either simultaneously with formation of the perforations or at a later time by any of a variety of methods. For example, the lower portion of the wellbore can be filled with a sand slurry prior to perforation. The sand is subsequently driven into the perforations and fractures by the pressured fluid in the wellbore during conventional overbalanced perforating operations.
As the high pressure pumps necessary to achieve an overbalanced condition in a well bore are relatively expensive and time consuming to operate, gas propellants have been utilized in conjunction with perforating techniques as a less expensive alternative to hydraulic fracturing. Shaped explosive charges are detonated to form perforations which extend through the casing and into the subterranean formation and a propellant is ignited to pressurize the perforated subterranean interval and propagate fractures therein. U.S. Pat. Nos. 4,633,951, 4,683,943 and 4,823,875 to Hill et al. describe a method of fracturing subterranean oil and gas producing formations wherein one or more gas generating and perforating devices are positioned at a selected depth in a wellbore by means of by a section of wireline which may also be a consumable electrical signal transmitting cable or an ignition cord type fuse. The gas generating and perforating device is comprised of a plurality of generator sections. The center section includes a plurality of axially spaced and radially directed perforating shaped charges which are interconnected by a fast burning fuse. Each gas generator section includes a cylindrical thin walled outer canister member. Each gas generator section is provided with a substantially solid mass of gas generating propellant which may include, if necessary, a fast burn ri20 ng disposed adjacent to the canister member and a relatively slow burn core portion within the confines of ring. An elongated bore is also provided through which the wireline, electrical conductor wire or fuse which leads to the center or perforating charge section may be extended. Primacord fuses or similar igniters are disposed near the circumference of the canister members. Each gas generator section is simultaneously ignited to generate combustion gasses and perforate the well casing. The casing is perforated to form apertures while generation of gas commences virtually simultaneously. Detonation of the perforating shaped charges occurs at approximately 110 milliseconds after ignition of gas generating unit and that from a period of about 110 milliseconds to 200 milliseconds a substantial portion of the total flow through the perforations is gas generated by gas generating unit.
U.S. Pat. No. 4,391,337 to Ford et al. discloses an integrated jet perforation and controlled propellant fracture device and method for enhancing production in oil or gas wells. A canister contains a plurality of shaped charge grenades around which is packed a gas propellant material so as to form a solid fuel pack.
U.S. Pat. No. 5,355,802 to Petijean describes a method and apparatus for perforating a formation surrounding a wellbore and initiating and propagating a fracture in the formation to stimulate hydrocarbon production from the wellbore. A tool includes at least one oriented shaped charge which is connected to detonator via a firing cord. At least one propellant generating cartridge is also positioned within tool and is connected to wireline cable through delay box via wires and cord.
U.S. Pat. No. 4,253,523 to Ibsen discloses a method and apparatus for well perforations and fracturing operations. A perforating gun assembly is comprised of a plurality of shaped charges positioned in spaced-apart relationship to each other in an elongated cylindrical carrier. The spaces in the carrier between the shaped charges are filled with a secondary explosive, such as an activated ammonium nitrate.
U.S. Pat. No. 5,005,641 to Mohaupt discloses a gas generating tool for generate a large quantity of high pressure gases to stimulate a subterranean formation. The tool comprises a carrier or frame having a series of staggered openings spaced longitudinally along the tubular member. Carrier receives a charge of propellant material which has a passage through which an ignition tube is inserted.
However, none of these prior art devices which utilized propellants in conjunction with perforating devices have proved to provide completely satisfactory results. Thus, a need exists for an apparatus and method for perforating and stimulating a subterranean formation which provides for improved communication between the wellbore and the subterranean formation penetrated thereby.
Thus, it is an object of the present invention to provide an apparatus and method for perforating and stimulating a subterranean formation which provides for improved communication between the wellbore and the subterranean formation penetrated thereby.
It is also object of the present invention to provide an apparatus for perforating and stimulating a subterranean formation which is relatively simple in design and can be readily employed with a variety of perforating gun designs.
It is a further object of the present invention to provide an apparatus for perforating and stimulating a subterranean formation which provides repeatable burns of the propellant component of the apparatus.
It is still a further object of the present invention to provide an apparatus for perforating and stimulating a subterranean formation which uses perforating charges of lesser energy than previously employed.
It is a still further object of the present invention to provide an apparatus and method for perforating and stimulating a subterranean formation wherein propellant is positioned between a perforating charge and the casing of a well bore.